Heisenberg's opposition to wave mechanics did not last long. Influenced by Bohr, he came to accept its elegant mathematical methods — though not Schrödinger's interpretation of them — and also the more concrete manner of presentation afforded by Bohr's Principle of Complementarity. In the preface to the lectures he gave at the University of Chicago in the spring of 1929 and published under the title The Physical Principles of the Quantum Theory, he speaks of the "conclusive studies of Bohr in 1927" on the nature of the quantum theory, and identifies himself completely with the Kopenhagener Geist der Quantentheorie founded, he says, upon the "complete equivalence of corpuscular and wave concepts". "What was born in Copenhagen in 1927", he wrote, "was not only an unambiguous prescription for the interpretation of experiments, but also a language in which one spoke about Nature on the atomic scale, and in so far a part of philosophy"¹. Heisenberg's acceptance of wave mechanics as an intrinsic part of the quantum theory was understandable; but his capitulation to the philosophy of complementarity was, in our opinion, unfortunate; for it led him away from the true sense of his original insight².